Molecular sieve crystals are generally microporous structures composed of either crystalline aluminosilicate, belonging to a class of materials known as zeolites, or crystalline aluminophosphates, or crystalline metalloaluminophosphates such as silicoaluminophosphates. The crystals are conventionally made by hydrothermal crystallization from a reaction mixture comprising reactive sources of silicon and/or aluminum and/or phosphorous containing compounds, usually in the presence of one or several organic amine or quaternary ammonium salts.
Molecular sieve catalysts are compositions made of molecular sieve crystal particles bound together to form a formulated catalyst material. The formulated molecular sieve catalyst composition typically includes other components such as binders, fillers such as clay, and optionally other catalytically active agents such as rare earth metal oxides, transition metal oxides, or noble metal components.
Conventional methods of making molecular sieve catalysts include mixing together molecular sieve and binder, as well as other optional components such as fillers and other catalytic components. The mixture is typically stirred in solution to form a slurry, and the slurry is dried to form molecular sieve catalyst particles. Following drying, the particles are calcined to harden, as well as to activate, the catalyst.
U.S. Pat. No. 4,764,269 (Edwards) discloses conventional methods of making and using SAPO-37 molecular sieve catalyst that can be used in catalytic cracking operations. The catalyst was found to be adversely affected by moisture, but the crystalline structure and activity of the molecular sieve component was preserved by including a stabilizing amount of the organic template compound used in the manufacture of the molecular sieve within the pore structure thereof until such time as the catalyst was thermally activated during use.
Metalloaluminophosphate molecular sieves, such as the SAPO-37 molecular sieve described by Edwards, have a variety of uses. A desirable characteristic for many of the metalloaluminophosphate molecular sieves, regardless of the process of use, is that the finished or formulated catalyst be attrition resistant, which can refer to hardness as well as ability to absorb shock, since the catalyst will typically have to endure severe stress in commercial scale processes.
For example, WO 99/21651 describes a method for making molecular sieve catalyst that is considered relatively hard. The method includes the steps of mixing together a molecular sieve and an alumina sol, the alumina sol being made in solution and maintained at a pH of 2 to 10. The mixture is then spray dried and calcined. The calcined product is reported to be relatively hard, i.e., attrition resistant.
U.S. Pat. No. 6,153,552 describes another method for making molecular sieve catalyst. The catalyst is made by mixing together a silicon containing oxide sol as a binder material and a molecular sieve material. The pH of the mixture is adjusted prior to spray drying. Following spray drying, the catalyst material is calcined to form a finished catalyst product, which is reported to be relatively hard, i.e., attrition resistant.
Attrition resistance continues to be a desirable characteristic in molecular sieve catalysts. As new process systems are developed, the ability of the catalyst to endure the stress of the process system is particularly important so as to increase the effective life of the catalyst in the reaction process. If the catalyst is not properly attrition resistant, it is likely to break apart at an early stage, meaning that the catalyst could only be effectively used for a relatively short period of time. Therefore, obtaining molecular sieve catalysts that have a high degree of attrition resistance are still sought.